A newapproach to the investigations of the stereochemistry of hydroxylation of primary carbon atoms is outlined. Specifically, we propose to define the origin (cycloartenol versus lanosterol) and the modes of oxygenation at C-19 and C-21 in the biosynthesis of cardenolides which are very important drugs in the treatment of cardiac patients. Hydroxylation of sterols at C-26 is a crucial step in the elaboration of bile acids by animals and in the degradation of the steroidal side chain by microorganisms. It is of interest that rat liver enzmyes and microbial enzymes oxygenate different C-25 pro-chiral methyls of sterols. We, therefore, propose to determine the mode of hydroxylation of the terminal methyls of sterols by the two enzyme systems.